Solar Geotechnical FAQ

Both geotechnical testing and pile testing are required to develop a solar project. Test pit excavations, test borings, in-situ field testing, and load testing are generally the most common practices prior to performing a geotechnical investigation for a solar project development.

Depending on the scope and budget of the project, the geotechnical investigation will consist of:

1. Soil type classification
2. Identifying Atterberg limits
3. Particle size distribution of soil
4. Friction angel or cohesion
5. Resistance factors for the various layers of soil
6. Corrosion considerations
7. Adfreeze (frost heave) and frost penetration depth (including historic weather correlation studies, percolation, and water table)
8. Pit excavation to find possible constraints during construction (soil stability, rocks, etc.)

A pile testing and design program is also necessary to determine how to best install the right solar racking foundation in the proposed site. The most important factors for a pre-production load testing and pile design testing program are the tension/pullout capacity and the lateral capacity. The parameters considered in a pile testing investigation include:

1. Testing loads
2. Testing piles
3. Axial compression test (only needed in very large, heavy solar systems)
4. Lateral test (not that necessary but optional)

A geotechnical report for a solar project usually includes results from the geotechnical investigation, the pile testing and design recommendations, and the pre-production load testing. The report begins with a basic background review of the site. It will then include information on the geotechnical investigation procedures such as the field investigation program, piezometer installation, laboratory testing program, and field resistivity testing.

Subsurface conditions are also detailed in length including information on the overburden, bedrock and other obstructions, and groundwater and borehole stability observations. Most if not all geotechnical reports conclude with a recommendations section which identifies general recommendations such as site preparation, excavations, groundwater control, and material reuse, backfill, and compaction.

The geotechnical recommendations section also includes frost considerations (such as soil frost susceptibility, frost penetration depths, and water availability), seismic site class, and recommendations for foundation design (including which pile is recommended such as driven piles or helical piles).

Other information that is found in a geotechnical report for a solar project includes the groundwater and soil stability, corrosivity analysis, thermal resistivity testing, liquefaction susceptibility, and other hydrogeological considerations such as an evaluation of impacts from sediment erosion control, contamination of aquifers, and/or the drawdown of water wells and water features. Construction monitoring and design review are also included.

Each geotechnical report will vary based on the scope of the project, which is why the initial site survey and test plan are so important. Depending on what is found in the initial screening, the geotechnical report will have different information and recommended designs.

Frost in the ground of your solar array can cause both frost heaving and adfreezing which causes irreparable damage to your solar racking foundation. While there is no way to prevent soil from freezing and melting, performing a cold weather-focused geotechnical and pile testing program can help you choose the correct foundation type and installation method. With the right foundation, frost heaving and/or adfreezing will not have any impact on your solar array.

Generally, there should be at least 5-8 boreholes per 5 MW of the general area of a proposed PV array. However, the exact number of geotechnical boreholes needed is site specific and depends on geotechnical factors such as the geographic location, soil type, groundwater, topography, etc.

Traditional geotechnical boreholes cover a very narrow but deep slice of the soil, whereas test pits cover a broader area of land despite being shallower. We recommend test pits in comparison to boreholes because they generally provide more information at a lower cost. Test pits are less intrusive to the site environment and do not require heavy machinery to operate. In effect, more of your budget can be spent on important subsurface testing to investigate the overall state of the soil in the proposed solar project area.

Wind loads are a type of environmental load caused by the force of the wind on the solar array. It includes uplift, shear, and lateral wind loads. Wind loads are an important consideration when designing solar racking foundations because of the widespread, flat characteristics of solar panels. The biggest issue with solar and wind is wind uplift, where wind pushes the solar panels up with extreme force. With the right foundation, a solar project can withstand winds higher than 100mph.

Seismic loads are a type of environmental load caused by the natural force of seismic activity. A solar array with sufficient a seismic load can resist severe damage from earthquakes. Depending on the geographic location of the solar array, the seismic load will have more or less importance.

Yes, there are tailored geotechnical investigation programs for solar projects. This is because soil and subsurface conditions vary significantly based on geographic location. All civil engineering and public works projects require geotechnical investigations to ensure the safety and reliability of the structure. Depending on the size and other factors identified in the preliminary site survey and test plan, the scope of the geotechnical investigation program for the solar project will vary.

Solar racking foundation design is the development of a uniquely engineered foundation that holds the solar panel racking and modules securely to the ground. Depending on the subsurface site conditions, the type of foundation required will vary. In general, there are three major types of solar racking foundations. They include:

1. Direct drive foundation posts
2. Helical posts, ground screws
3. Concrete ballasts

Frost depth design is a geotechnical practice of designing structures and foundations that extend far enough below the frost line so that they are not affected by frost upheave and/or adfreeze. The frost line is the depth at which groundwater in the soil freezes.

Geotechnical borehole logs are a record of the layers of soil and rock found in the various boreholes. Borehole logs can be further expanded upon in a laboratory setting to determine more specific characteristics related to lithology and hydrogeology.

A solar racking dead load (or permanent, static load) consists of the total weight of all the materials and attachments that are fixed to the racking structure. It is relatively constant over the entire lifetime of the solar array. Due to the lightweight nature of solar racking and PV modules compared to other large civil structures, the racking dead load is minimal compared to more impactful forces such as axial and lateral loads that are generated from frost, snow, and wind.

Snow loads are a type of environmental load caused by the force of snow on the solar array. The larger the solar array, the more likely snow will have an impact on the solar structure. Thus, most solar arrays in geographic regions that experience a winter must withstand a certain snow load threshold. Despite this, the angle that your solar arrays are set at will have a significant impact on their ability to deal with snow loads. At particular angles, snow will fall off the panels very easily.

Frost heave occurs when frost penetrates the ground under the foundation footing and water is pulled up from beneath the frozen soil. Once it reaches the freezing zone it solidifies, forming layers of ice – or lenses. These lenses force soil particles to separate and causes the soil surface to heave upwards. Adfreeze is a similar condition where frost heave occurs next to a foundation wall instead of under the foundation footing. When the ice lens melts in the spring, water falls back deep into the soil and a void is created by the melted lens. Adfreeze causes a solar racking foundation to drop to a lower elevation than it was before the frost occurred.